mkinitcpio generates init image with init_functions containig this:

device=$(blkid -t "$root" -o device)

But if I have multiple filesystems with same LABEL or UUID, then blkid would return all of them and it will break $device setting and leave system unbootable. Yes, I have to identical filesystems. I use them for backup purpose.

Example of calling blkid:

# blkid -t "LABEL=test" -o device
/dev/loop0
/dev/loop1

To fix this, one should add option "-l" to blkid:

# blkid -l -t "LABEL=test" -o device
/dev/loop1